1. Field
This disclosure relates to a probe inspection apparatus for testing a flat panel display and, more particularly, to a probe inspection apparatus capable of measuring a change in the electrical properties of circuit patterns formed in the thin film transistor (hereinafter referred to as a ‘TFT’) substrate of a liquid crystal display.
2. Related Art
A variety of flat panel displays (FPDs) which are capable of reducing the disadvantages of a cathode ray tube (i.e., weight and volume) are recently emerging. The flat panel displays may comprise a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an electro-luminescent display, and so on. There is a tendency that, from among the displays, the liquid crystal display gradually finds various applications because of its light weight, slimness, low-power driving, etc.
A process of fabricating the liquid crystal display comprises a process of fabricating a rear substrate, a process of fabricating a front substrate, a process of coalescing the front and rear substrates, and the like. The front substrate comprises a mother substrate formed of a plurality of color filter substrates, and the rear substrate comprises a mother substrate formed of a plurality of TFT substrates.
In the TFT substrate, a plurality of horizontal lines and a plurality of vertical lines are formed to cross each other in a matrix form, and pixels, each having a transparent pixel electrode, are formed at the respective intersections of the vertical lines and the horizontal lines. TFT each coupled to the vertical line, the horizontal line, and the pixel electrode are formed in the respective pixels. The rear substrate in which the plurality of TFT substrates is formed experiences a test process and then coalesces with the front substrate. The coalesced front and rear substrates are cut into liquid crystal panels through a scribing process. A liquid crystal panel, a backlight unit, etc. are assembled in each liquid crystal panel, thereby constituting a liquid crystal module. Driving circuits are coupled to the liquid crystal module, thereby completing a liquid crystal display.
In a test process for the rear substrate during the process of fabricating the liquid crystal display, whether a circuit pattern (for example, a TFT) formed in the rear substrate is good or bad is determined by supplying an electrical test signal to the circuit pattern using a probe inspection apparatus. This circuit pattern test may be performed for the TFT substrate cut from the mother substrate (i.e., the rear substrate), but is generally performed for the mother substrate (i.e., the rear substrate) in order to increase the efficiency of a test. Hereinafter, the mother substrate (i.e., the rear substrate) is referred to as a substrate, for convenience of description.
In order to obtain a more accurate measurement value when the electrical properties of a circuit pattern are tested, the probe inspection apparatus requires a temperature and ambient condition similar to the actual operating condition of a circuit pattern in the liquid crystal module state. That is, the probe inspection apparatus is required to operate under such conditions as light and heat generated by the back light while the liquid crystal display operates.
The influence of light when a change in the characteristics of a circuit pattern is measured is described below. The electrical properties of a circuit pattern formed in the substrate are greatly influenced by a photo current. Here, the photo current refers to an electric current generated when the flow of an electric current of the circuit pattern is changed owing to a photoelectric effect in the case where the circuit pattern formed in the substrate is exposed to light. The photo current of a circuit pattern, generated by an illuminator such as a fluorescent lamp, is very different from that of a circuit pattern, generated by back light in the liquid crystal module state. Further, the luminance and color temperature of an illuminator, such as a fluorescent lamp, is very different from that of a back light. Accordingly, measurement information about the electrical properties through a conventional probe inspection apparatus is inevitably being used as only reference information. Since precise process management and measurement become necessary with the larger size of a liquid crystal display, a change in the photo current which had not been conventionally problematic has a great effect on the quality of a product. Accordingly, an illuminator capable of performing the same function as the back light is being required in a probe apparatus (i.e., an apparatus for testing the electrical properties of the substrate).
The effect of heat when a change in the characteristics of a circuit pattern is measured according to the application of an electrical signal is described below. As described above, in order to obtain more precise measurement values when the electrical properties of a circuit pattern formed in the substrate are tested, the same heat condition as that when a liquid crystal display operates has to be provided. Accordingly, a hot plate is conventionally provided in a probe inspection apparatus so that more precise measurement values can be obtained while the same temperature condition as that when the liquid crystal display operates is maintained. If the hot plate is used, however, not only noise is generated when the circuit pattern is heated, but also it is difficult to control a heating temperature for maintaining a change in the measurement temperature within a specific range. In other words, if it is sought to measure the heat characteristic of the circuit pattern, a very low electric current flowing through the circuit pattern has to be measured (10E-15A) while heat is applied to the circuit pattern by slowly (generally, −10 V to +30 V) raising voltage of the hot plate. It is, however, impossible to precisely measure a change in the heating characteristic of the circuit pattern due to noise generated upon heating because an electrical heating method is used. Further, in order to precisely measure the heating characteristic of the circuit pattern, a change in the measurement temperature has to be very small. However, a test apparatus using the above-described hot plate is difficult to control the change in the measurement temperature. This hot plate cannot be used to measure a change in the characteristic caused by light because it is formed of an opaque metal plate.
Consequently, since more precise process management and measurement for a liquid crystal display are required, a need to measure the electrical properties for the substrate in a state similar to that in which the liquid crystal display actually operates (i.e., a state in which heat and light are affected at the same time) comes to the forefront. However, the conventional probe inspection apparatus using the hot plate and the illuminator can be used to perform measurement using only heat or light. Further, the conventional probe inspection apparatus is problematic in that the reliability of measurement data and the convenience of measurement are low because a change in the characteristic resulting from heat and a change in the characteristic resulting from light are separately measured and the two characteristic change data are combined and used.
Moreover, the conventional probe inspection apparatus is used to measure only the electrical properties of a TFT in the state in which the TFT has been completed. Accordingly, the conventional probe inspection apparatus has problems in that a change in the characteristic of a TFT depending on a change in the channel width (W)/channel length (L) of the TFT (i.e., critical factors in an actual characteristic) cannot be known and characteristic analysis is therefore limited because it cannot measure a change in the W/L of a channel during a test process.